[alsa-devel] [PATCH 2/4] topology: Change variable type to fix gcc warning

[Takashi Iwai]

On Wed, 18 Nov 2015 16:14:43 +0100,
Lin, Mengdong wrote:
> 
> > -----Original Message-----
> > From: Takashi Iwai [mailto:tiwai at suse.de]
> > Sent: Wednesday, November 18, 2015 4:19 PM
> > 
> > On Wed, 18 Nov 2015 08:28:09 +0100,
> > Lin, Mengdong wrote:
> > >
> > > > -----Original Message-----
> > > > From: Takashi Iwai [mailto:tiwai at suse.de]
> > > > Sent: Wednesday, November 18, 2015 3:16 PM
> > > > To: mengdong.lin at linux.intel.com
> > >
> > > > On Wed, 18 Nov 2015 08:23:07 +0100,
> > > > mengdong.lin at linux.intel.com wrote:
> > > > >
> > > > > From: Mengdong Lin <mengdong.lin at linux.intel.com>
> > > > >
> > > > > Fix warning: comparison between signed and unsigned integer
> > > > > expressions [-Wsign-compare]
> > > > >
> > > > > ABI objects use type _le32, which is converted to host unsigned integer.
> > > > > So the iterator 'i' in a loop as below should also be unsigned.
> > > > > for (i = 0; i < pcm->num_streams; i++)
> > > > >                 ^
> > > >
> > > > Using an unsigned int for the generic loop count like i is strange.
> > >
> > > Yes.
> > >
> > > > Rather compare with the original value in the template, which is actually int.
> > >
> > > Are you suggesting not change the code?
> > 
> > No.
> > 
> > > The "pcm->num_streams" here is __le32 defined in ABI:
> > > struct snd_soc_tplg_pcm {
> > > 	...
> > > 	__le32 num_streams;	/* number of streams */
> > > 	...
> > > } __attribute__((packed));
> > >
> > > It seems gcc takes it as unsigned integer and so I get the warning.
> > 
> > The problem isn't only about the endianess.
> > Conceptually, it's wrong to refer directly to le32 data, as it's not being CPU
> > endian.  It should have always an endian conversion.  Of course, we support
> > only LE32, so it's no big issue, so far.
> > 
> > What you need to refer to is the value in the template instead.  It's guaranteed
> > to be CPU endianess, and it's even int.
> > So, the code would look like:
> > 
> > 	for (i = 0; i < pcm_tpl->num_streams; i++)
> > 
> Okay, I'll revise the patch. Thanks for your suggestion.
> 
> > BTW, I haven't checked whether topology API would "work" on big-endian
> > architecture; does it return an error properly?
> 
> Sorry, no. I'll check if an API can tell us the host endianness and let topology return an error for big-endian machines.
> 
> I had thought to use this API when generating the ABI objects from host type to _le32:
> uint32_t htole32(uint32_t host_32bits);

Yes, that should work.  But then, as I pointed out, many current codes
directly refer to the converted data, and they are broken.

> But since I have no big-endian machines to test this, maybe it's better to return an error for them.

It's the easiest option, of course :)


Takashi